The present invention relates to lithium batteries and, more particularly, to lithium batteries having depletion gauges or indicators.
Lithium batteries have become well-known for their long life and suitability as power sources for cardiac pacers and nerve stimulators. Normally these batteries utilize a lithium anode, a cathode formed of cupric sulfide or silver chromate, and an electrolytic solution.
With the long life associated with such batteries and due to the fact that such batteries are generally implanted within the human body, it is desirable to have a method for indicating that a predetermined level of discharge has occurred. This is particularly important in life sustaining devices, such as cardiac pacers, in order to provide adequate time for the removal of the power source before it fails.
An important feature of the present invention is that of providing a means within a battery for providing a positive indication when a predetermined level of discharge has occurred, or conversely, to provide a positive indication when the remaining life of a battery has reached a predetermined level.
It is important that the positive indication of remaining battery life be provided regardless of the type of service the battery has been providing. In addition, it is important that the depletion indicator be compatible with the present construction and configuration of lithium batteries utilized in implanted devices such as cardiac pacers and nerve stimulators.
In certain types of lithium batteries, the internal resistance of the battery increases linearly with the discharge of the cell. With this type of battery, the output voltage provides a satisfactory indication of the remaining life of the cell.
With certain lithium batteries used in cardiac pacers, the voltage output remains approximately constant until the cell is completely discharged. With other types of lithium batteries, such as the lithium-cupric sulfide battery, an end-of-life indication may be obtained by taking advantage of the two stages of discharge i.e.: EQU Cu.sup.+2 .fwdarw.Cu.sup.+1 E=2.1 volts EQU Cu.sup.+1 .fwdarw.Cu.sup.o E=1.7 volts
By adjusting the ratio of lithium to cupric sulfide, it is possible to control the ratio of these two reactions. Accordingly, when the transition from the higher voltage to the lower voltage occurs, this change in voltage provides an indication that a predetermined level of discharge of the cell has occurred. In batteries used with cardiac pacers, the length of the second voltage level is adjusted to be approximately 10 percent of the first voltage level. One problem with the dual voltage level system is that once the second level is obtained, the battery may then last from three months to one year depending upon the load applied to the battery at the lower voltage level.
With cardiac pacers of more modern design which include complex programmable capabilities, the drain on the battery may vary over a broad range. It, therefore, becomes desirable to have an additional means for evaluating the residual capacity of the battery at a time early enough to establish orderly plans for the replacement of the device. Also, as greater loads are placed upon batteries, the effectiveness of the dual level indicator system is substantially reduced.